The present invention relates to a method for the rheology control of fluid phases and a suitable composition for this purpose.
The thickening of water- and oil-based systems, so-called rheology control, is carried out in practice especially with the aid of finely divided swellable clays and/or other silicate compounds of natural or synthetic origin. The various fields of work make use of the possibility, existing here, of the shear-thinning and/or thixotropic thickening of the respective fluid phases.
In particular, adducts of mixed metal hydroxides and clay have been widely described in the past and are popular in practice. The individual mixed metal hydroxide types each thicken the initially introduced clay suspension, with the result that pronounced shear-thinning rheology is obtained.
Mixed metal oxides (xe2x80x9cMMHxe2x80x9d) or layered double hydroxides (xe2x80x9cLDHxe2x80x9d) are a class of substances comprising clay-like minerals of the general empirical formula
[MII1xe2x88x92xMIIIx(OH)2](An)xe2x88x92x/n
Here, MII and MIII are divalent or trivalent metal cations and A is any desired anion. A further designation, namely xe2x80x9cHTC-type mineralsxe2x80x9d, is derived from the most well-known member, hydrotalcite, an Mgxe2x80x94Al-carbonate-MMH.
The preparation of MMH compounds by coprecipitation and their use as thickeners are described, for example, in the patent documents European Patent 207 801 and European Patent 207 811. Accordingly, an Mg Al hydroxychloride MMH having the composition MgAl(OH)5-xClx is obtained, for example, by precipitation of MgCl2 with AlCl3 in the presence of a base.
Owing to the various disadvantages of this coprecipitated MMH product, such as, for example, high production costs and the necessity of a dispersant, an improved material in the form of the activated, hydrated mixed metal oxide (AHMMO) was made available to the user. A mixed metal oxide or mixed metal oxyhydroxide of the general empirical formula
Mg1xe2x88x92xAlx(O)y(OH)z
is obtained by thermal activation of, for example, hydrotalcite. The activation process, optimized with respect to the thickening effect, results in the compound additionally containing smaller residual amounts of carbonate and water of hydration. The commercial product thus obtained is a highly effective MMH concentrate. If this product is added to water, it undergoes hydration with formation of Mgxe2x80x94Alxe2x80x94OH-MMH (AHMMO). This MMH species having mobile hydroxide moieties instead of carbonate is suitable, according to European Patent 539 582, as a thickener for aqueous clay suspensions. In contrast to the coprecipitated MMH, the AHMMO is chloride-free, requires no additional dispersant and can be prepared in a more economical and environmentally compatible manner.
According to European Patent 617 106, however, mixtures of, for example, sodium aluminate and magnesium oxide also act as thickeners in aqueous clay suspensions. As various analytical methods, in particular X-ray diffraction, have shown, Mgxe2x80x94Alxe2x80x94OH-MMH according to the empirical formula presented at the outset form here again in situ.
In addition to these MMH or LDH types having a layer structure, the mixed metal hydroxides having a three-dimensional network structure are also used for thickening aqueous clay suspensions. In WO 94/02 566, katoites having the basic formula
Ca3Al2(OH)12
in which some of the OH groups are replaced by silicate radicals, are mentioned as being preferred for this purpose. Such MMH compounds are also known under the name mixed metal silicates (MMS). However, some MMS/clay drilling fluids have a substantially lower thermal stability than MMH/clay-based fluids.
The Theological properties of MMH/clay-based drilling fluids are very valuable in particular for drilling technology.
Auxiliary fluids thickened to a shear-thinning viscosity of a greater or lesser extent are preferably used in the technology of geological and other drilling operations in the earth, but also, for example, as an earth support in excavations, in particular in subterrain curtain construction, in the sinking of shafts, wells and caissons, in pipe forcing, etc. Particularly important fields of use are wells for petroleum or natural gas exploration and horizontal drilling for trenchless pipe construction.
Drilling fluid systems which are sufficiently thickened by the addition of mineral viscosifiers without losing their flowability and pumpability under shearing stress and which contain additional dissolved, emulsified and/or suspended assistants adapted to the respective situation are widely used.
The high carrying capacity of MMH/clay fluids has proven its worth especially in large-caliber wells, when milling out damaged casing, in horizontal drilling and in drilling through coarse gravel. In particular, stuck-pipe problems by the settling of drill cuttings are prevented. In addition, the drilling fluid should be of low viscosity and readily pumpable at points of higher shearing stress, such as, for example, on emergence from the drill bit. For rapid advance of the drilling, a reduction in viscosity, which is high at rest, with growing shear gradient is required. This type of flow behavior is generally referred to as shear-thinnings.
MMH/clay-based drilling fluids have such a rheology. In contrast to biopolymers having also a shear-thinning effect, such as, for example, xanthan gum, crosslinking with bentonite, a smectite clay, usually used in drilling technology takes place when MMH is used. This cooperation of MMH with bentonite in the common network results in extreme shear-thinning fluid properties at relatively low costs in comparison with the biopolymer drilling fluids. The latter must in fact generate the desired rheology completely by themselves, for which purpose substantially higher doses are required.
According to Bingham, the rheology of a drilling fluid can be described by the yield point (YP[lbs/100 ft2]) and the plastic viscosity (PV[cP]). These parameters can be determined by measuring the shear stress in a rotational viscometer (e.g. FANN 35 from Baroid, Houston, USA) at different shear rates. Thus, plastic viscosity PV is obtained as the difference in shear stress at 600 and 300 rounds per minute, and the yield point YP as the difference between the PV and the shear stress at 300 revolutions per minute. The respective yield point is however always proportional to the carrying capacity of a drilling fluid. However, it should be noted that a high plastic viscosity results in only a small rate of penetration. A typical shear-thinning rheology is characterized by low PV and high YP values.
According to the prior art to date, MMH/clay, MMS/clay and biopolymer drilling fluids are not suitable for high-temperature applications at  greater than 300xc2x0 F. ( greater than 149xc2x0 C). Biopolymers lose their activity at the latest at about 280xc2x0 F. (about 138xc2x0 C.). High-quality AHMMO compounds, too, are limited to temperatures of use of up to and including about 300xc2x0 F. (about 149xc2x0 C.).
It was thus the object of the present invention to provide a method for the rheology control of fluid phases and suitable compositions therefor, which are suitable both for water-based and for oil-based systems and cover a wide temperature range.
This object was achieved by a method in which adducts of layered mixed metal hydroxides (MMH) and smectite clays are used as rheology control compositions, a hectorite being at least partly used as the smectite clay. Hectorite is a material having the approximate composition Na0.33 (Mg,Li)3[Si4O10](OH, F)2, optionally without Li and/or F. The hectorite structure derived from the prototype talc is described, for example, in Ullmann""s Encyclopaedia of Industrial Chemistry, 5th Edition, Vol. A7, pages 110-111. The proportion of hectorite in the total amount of smectite clay is preferably at least 20% by weight and particularly preferably at least 50% by weight.
Surprisingly, it has been found that, with the method according to the invention, the corresponding composition, not only is the desired broad field of use as a rheology control composition in both water-based and oil-based fluid phases covered but also reliable rheology control is permitted in high-temperature applications. These advantages were not to be expected on the basis of the experience to date with smectite clays.
MMH components which have proven suitable for the present method and the adducts used therein are those which have in particular the formula (I):
MmDdT(OH)(m+2d+3+nxc2x7a)Ana.qH2Oxe2x80x83xe2x80x83(I),
in which
M=at least one monovalent metal ion,
m=0 to 1,
D=at least one divalent metal ion,
d 0 to 6,
T=at least one trivalent metal ion,
A=at least one monovalent or polyvalent anion which differs from OHxe2x88x92,
a=number of anions A,
n=valency of the anions A (and hence a negative number),
nxc2x7axe2x89xa60,
(m+d)  greater than 0,
qxe2x89xa70
and
(m+2d+3+nxc2x7a)xe2x89xa72.
The compound (I) may contain a trivalent cation T but also different trivalent cations T, e.g. Al3+ and Fe3+, whose stoichiometry sums to T. The symbols m, d and a may denote both integers and fractions. Owing to the xe2x80x9copenxe2x80x9d layer structures of the mixed metal hydroxides, it is usually not possible to state a preferred range for the number q of water molecules.
Particularly preferably used mixed metal hydroxides are those of the formula (II)
[MII1xe2x88x92xMIIIx(OH)2](An)xe2x88x92x/n.qH2Oxe2x80x83xe2x80x83(II),
in which
MII=Ca, Mg, Zn, Cu, Ba, Sr, Fe, Ni, Mn and/or Co,
MIII=Al, Fe, Co, Ni, Mn, Cr and/or Ga,
A=monovalent and/or polyvalent anions having the valency n less than 0 and preferably hydroxide, halide, sulfate, nitrate, carbonate, silicate, phosphate and/or borate,
x=0.2 to 0.5,
qxe2x89xa70.
Specific fields of use may necessitate the use of specially prepared MMH, which the present invention also envisages, and for which MMH which were produced by hydration of mixed metal oxides and/or mixed metal hydroxides, which may in turn be thermally activated, are used.
It is advisable to use, as precursors of the mixed metal hydroxides activated hydrotalcites of the general formula (III)
Mg1xe2x88x92xAlx(O)y(OH)zxe2x80x83xe2x80x83(III)
in which
x=0 to 1,
y=0 to 1.5,
z=0 to 3, with
x=2y+zxe2x88x922,
which can be activated by the action of heat. For this activation, in general temperatures of from 400 to 700xc2x0 C. are sufficient for converting the metal compounds into their correspondingly dehydrated oxide or oxyhydroxide form (MMO) by expulsion of water and/or CO2.
According to the present invention, it is however also possible to use, in addition to the MMH described above, which were produced from MMO, also those mixed metal hydroxides which are referred to as so-called coprecipitates and in general have the formula (IV)
xe2x80x83MgAl(OH)5xe2x88x92yCly.qH2Oxe2x80x83xe2x80x83(IV)
in which y=0 to 2 and
q=an unspecific number of molecules of water of hydration.
Owing to the xe2x80x9copenxe2x80x9d layer structures of the coprecipitates, it is usually not possible to state a preferred range for the number q of water molecules.
In this case of the coprecipitates, too, it is possible to use MMH forms which have been thermally activated beforehand.
The abovementioned wide range of use of the method according to the invention is additionally documented by the fact that the adducts used according to the method may also contain mixed metal hydroxides which are not added as such to the fluid phase but are formed in situ, which is particularly preferably effected by using the starting compounds which contain the respective components MII and MIII in salt or oxide form, or any desired mixtures thereof, such as MgIIO/NaAlIIIO2, in the corresponding amounts and optionally in the presence of a suitable base. The combination of MgO/FeSO4, in which the iron ion passes through a valency change from II to III may be mentioned as a further example of a suitable mixture of components MII/MIII.
A further important advantage of the method according to the invention is that the adducts used for this purpose preferably have an MMH/clay or hectorite ratio of from 0.01 to 1:1 and particularly preferably a ratio of from 0.05 to 0.2:1, based on weight, which once again illustrates the wide applicability of the method in numerous fluid phases having a very wide range of properties and compositions.
In this context, the method according to the invention also takes into account specific circumstances in which the addition of the MMH/hectorite adducts alone is no longer sufficient for influencing the fluid phase in the desired manner. In these cases, the MMH/hectorite adducts may then also be used together with other rheology control compounds, for example in particular water-soluble and/or water-swellable polymer compounds of natural and/or synthetic origin, the use of thermally stable polymer compounds being particularly preferred.
At this point, nonionic, weakly anionic, weakly cationic or zwitterionic polymers may be mentioned by way of example, which polymers, in contrast to highly anionic or cationic polymers, do not adversely affect the MMH/clay adduct formation, and the addition of which results in a reduction of filtrate water undesired in drilling technology. In particular, synthetic betaine-based copolymers and terpolymers are suitable.
In practice, it may also be necessary to change the proportion of hectorite in the adducts. The method according to the invention therefore also takes into account the variant of using adducts whose clay fraction contains up to 80% by weight of clays which differ from hectorite. All suitable clays, in particular of the smectite type, such as corresponding talc or pyrophyllite types, may be used for this purpose but in particular bentonite has proven a suitable second clay component in addition to hectorite.
As already mentioned several times and demonstrated by the variety described above, the method according to the invention is suitable for wide use for influencing the rheology in fluid phases. The method develops its positive properties particularly when combinations of MMH and hectorite or MMH/clay mixtures are used in drilling fluids.
Regarding the use in drilling fluids, it should be mentioned that the method is suitable in particular for applications in the high temperature range, and it is for this reason that the invention particularly recommends the use of adducts at temperatures  greater than 300xc2x0 F., i.e. in regions  greater than 149xc2x0 C.
In addition to the method for the rheology control of fluid phases, the present invention also relates to a composition which is particularly suitable for this purpose and which contains adducts of layered MMH and hectorite. In addition to hectorite, the composition may also contain further clays, so that the invention also relates to adducts whose clay fraction contains up to 80% by weight, preferably up to 50% by weight, of clays differing from hectorite, especially bentonite.
By means of the claimed method and the composition particularly suitable for this purpose, it has been possible in particular to improve the thermal stability of shear-thinning drilling fluids so that the rheologies of the fluid phase(s), as required, for example, in the exploitation of deep petroleum and natural gas deposits or in the drilling of geothermal wells, withstand thermal loads  greater than 300xc2x0 F. ( greater than 149xc2x0 C.) without problems.
The Examples which follow are intended to illustrate the advantages of the method according to the invention and of the composition according to the invention, in particular when they are used in the high temperature range.